It is known to fabricate a fleece or mat of thermoplastic polymer filaments in the form of a web in which the filaments are collected in the form of a mat with point bonds or wells between the filaments where they cross one another.
The polymers used can be of the two-state type described, i.e. on a supermolecular level can be composed of crystallites imparting a crystallitic state to the polymer or can be in an amorphous state or some combination of the two, the degree of crystallinity being a measure of the proportion formed by crystallites.
Polymer filaments refer to strands of considerable length, for example, endless strands and monofilaments. They are to be contrasted with polymer fibers which are of relatively short lengths, generally referred to also as staple fibers. Suitable polymers for the purposes described herein are polyamides, polyethylene, polyesters and polypropylenes, although the invention is not to be considered limited to these polymeric materials although it is essential for the purposes of the present invention that the polymer used have the two states described. Especially stable are polyamide 6 (Nylon 6), polyamide 6.6 (Nylon 66) and polyethyleneterephthalate. The dominating parameters of the crystallitic state are the chain packing in the crystal structure, the degree of crystallinity, the crystallite orientation and the crystallite size. With such polymers, however, it is found that the chain packing in the crystallite structure can generally not be influenced by the processing conditions of the polymer. The degree of crystallinity and especially the crystallite orientation can be influenced by the processing.
Since the crystallite structure is especially stable, the molecular chains do not tend to fold back on themselves. The shrinkage of the polymers decreases with increasing degree of crystallinity. The crystallite component has an effect on strength of the filaments only insofar as the crystallite orientation extends along the filament axis. The degree of crystallinity decreases with increasing cooling speed. The higher degree of orientation of the molecular chains in the crystal structure results in a promotion of crystallinity. The term "orientation" in this sense refers not only to the orientation of the molecular chains in the amorphous region but also the orientation of the crystallites in the crystallite region. Upon stretching of the filaments, the molecules and the crystallites tend to orient in the direction of stretch. The degree of orientation depends strongly upon the thermal and mechanical stretching conditions. The stretching conditions which should be applicable can be easily determined empirically once the desiderata for such conditions are known. With increasing orientation the filament strength increases with a simultaneous reduction in the elongation and the shrinkage ratio. In the melt the molecular chains are without specific orientation and tends to be jumbled (compare ITB Garn- und Fl achenherstellung 2/94, pages 8, 9).
Processes for producing a mat or fleece of thermoplastic polymer filaments are known in that a variety of techniques (see, for example, U.S. Pat. Nos. 4,340,563, 4,405,297, 3,855,045, 5,296,289, German patent 4,014,414 and German Patent 4,014,989).
The polymer filaments result from a polymer melt which is fed through nozzle orifices of a so-called spinerette to form a filament curtain. The filament curtain passes through a cooling chamber together with process air and the filament curtain then traverses a stretching passage in which stretching of the polymer filament is effected by entrainment air, e.g. by forcing the air through a constricted region which increases its velocity. The stretched polymer filament is collected on a continuously movable sieve belt to form the mat or web and generally the deposition of the polymer filament upon the belt is assisted by the application of suction below the belt.
Upon the deposition of the polymer filament on the belt, random cross-overs between the filaments results in contact points at which the crossing filaments are welded together to form the coherent web. These crossing weld points are distributed in the longitudinal direction of the web and transversely across the web.
The web can be heated (German patent 1,900,265) to a stretching temperature and then stretched both in the longitudinal direction and in the transverse direction (i.e. biaxially). The biaxially stretching can result in a reduction of the weight per unit area of the web. It is also known (see U.S. Pat. No. 5,296,289) to calender the web to ensure point weld structures between the crossing polymer filaments with diameters of the millimeter range. For this purpose the web can be passed between the pair of calender rolls and generally at least one of these rolls is heated. The degree of crystallinity of the polymer filaments determines largely the physical parameters of the polymer filaments in the mat and thus the physical parameters of the mat itself.
While nonwoven fabric webs of this type are widely used at the present time for a large variety of materials for liquid take-up purposes, as insulation material, as linings, as fillings and even as fabrics for covering or lamination to other materials, there nevertheless is a desire to improve the strength of such materials for a given weight per unit area or, conversely, to reduce the weight per unit area for a given strength of the web.